By the mode in which the driving force generated by the vehicle engine is transferred to the driving wheels, vehicles are classified into two-wheel drive vehicles and four-wheel drive vehicles.
Here, the two-wheel drive vehicles are classified into Front engine Rear drive (FR) mode and Front engine Front drive (FF) mode by the location of the driving wheels, namely the rear wheels or the front wheels that actually drive the vehicle.
The driving force generated by the engine of a four-wheel drive vehicle is transferred to the front wheels and the rear wheels at the same time to drive the whole vehicle and improve the stability when the vehicle runs on a road on a rainy or snowy day or on an icy road, or enhance the vehicle driving performance when the vehicle runs on an off-road surface such as gravel road surface or rugged road surface.
Currently when the four-wheel drive system for the off-road purpose is used to transfer the driving force, since the driving force is directly transferred between the front wheels and the rear wheels and the rotation difference between the wheels cannot be offset when the vehicle turns a corner, the vehicle cannot make a turn as desired by the driver. This phenomenon is called “Tight corner braking”.
To solve the aforesaid problem in the full-time four-wheel drive system for the on-road purpose, a multi-disk clutch is set between the front wheels and the rear wheels. If a rotation difference occurs between the wheels when the vehicle makes a turn or the road surface condition changes, the multi-disk clutch is driven to distribute the driving force between the front wheels and the rear wheels and offset the rotation difference between the wheels, thus improving driving safety of the vehicle.
However, the Transfer Case Control Unit (TCCU) responsible for distributing the driving force to the four wheels in the full-time four-wheel drive system cannot distinguish the inter-wheel rotation differences when the vehicle makes a turn or runs on a slippery road surface from the inter-wheel rotation differences caused by the air pressure difference among the tires. When the inter-wheel rotation differences occur due to the air pressure difference among the tires, the TCCU also thinks the rotation differences are arising from making a turn or a road surface condition change, and will then implement the drive control on the four wheels.
Owing to the aforesaid disadvantage, the multi-disk clutch may also operate continuously in a situation where it should not operate, which leads to a failure of the four-wheel drive system.